Board-on-Board Fence Calculator

Board-on-Board vs. Conventional Fence Design

A standard picket fence spaces vertical boards at intervals along horizontal rails, deliberately creating gaps for visual openness. Board-on-board construction eliminates these gaps by overlapping each picket partially over its neighbour, creating a solid visual barrier.

The key advantage emerges over time. Wood shrinks as moisture content drops—typically 12% of its width in the worst-case scenario. A conventional fence with tight initial spacing will develop gaps as pickets narrow. Board-on-board design preempts this problem: the overlap ensures that even after shrinkage, the second layer remains visible and maintains the privacy seal. This layering technique is why board-on-board fences remain popular for properties requiring year-round screening.

Board-on-Board Fence Material Calculations

Posts, rails, and sections follow the same calculations as conventional fences. The distinction lies in how picket quantity accounts for the overlapping offset:

• Fence Length — Total horizontal distance the fence spans
• Post Spacing — Distance between consecutive posts (typically 6–8 feet)
• Fence Height — Vertical measurement of the installed fence
• Post Length — Total length of each post; must be 1.5× fence height to allow 1/3 burial depth
• Picket Width — Horizontal dimension of each board
• Overlap — Amount each picket covers the adjacent one (accounts for future shrinkage)
• Rails per Section — Number of horizontal support beams per fence panel (commonly 2)

Choosing the Right Overlap

Wood shrinkage varies by species, climate, and initial moisture content. A practical rule is to apply an overlap equal to approximately 24% of the picket width. This buffer absorbs worst-case shrinkage (12%) and ensures a safety margin.

Real-world examples help guide selection:

• 3½-inch pickets: Use 0.8 inches of overlap
• 5½-inch pickets: Use 1.3 inches of overlap
• 11½-inch pickets: Use 2.8 inches of overlap

For standard 8-foot sections, a 5½-inch picket with 1½-inch overlap yields approximately 24 pickets; a 3½-inch picket with 1-inch overlap requires about 38 pickets. Wider pickets mean fewer boards to cover the same distance but demand proportionally larger overlaps.

Common Pitfalls and Installation Considerations

Overlooking these details during planning or installation can compromise both aesthetics and longevity.

1. Underestimating post burial depth — Posts must sit at least one-third underground for stability. A 6-foot fence requires a 9-foot post. Shallow installation invites leaning or toppling, especially in loose soil or high wind zones.
2. Ignoring local soil and frost conditions — Frost heave pushes shallow posts upward in freezing climates. Set posts below the frost line (which varies regionally from 2 to 4 feet) to prevent this movement from misaligning your carefully planned picket pattern.
3. Miscalculating overlap for future movement — Using less than 20% overlap risks visible gaps after one or two seasons. Climate zones with extreme wet-dry cycles demand the full 24% rule; drier regions may tolerate slightly less, but erring toward caution avoids later regrets.
4. Mixing wood species or moisture content — Different lumber species shrink at different rates. Pressure-treated and untreated boards also behave differently. Consistency in sourcing prevents unpredictable gaps or warping that no overlap can fully remedy.

Post and Section Layout Fundamentals

The number of posts exceeds the number of sections by one. A 40-foot fence with 8-foot spacing needs 6 sections and 7 posts. Posts anchor the horizontal rails; rails support the pickets.

Rails typically number 2 per section in residential applications, though 3 or 4 may be used for taller fences (8 feet or more) to resist wind load. Each additional rail creates a sturdier frame but increases material cost. Spacing posts evenly matters both functionally and visually—uneven gaps highlight construction errors and can weaken structural integrity at wider sections.